1. Field of the Invention
The present invention relates to the allocation of resources to transmit uplink packet data in an Orthogonal Frequency Division Multiplexing (OFDM) system. More particularly, the present invention relates to a method and apparatus for allocating resources by considering frequency scheduling gain and frequency diversity gain when uplink packet data is transmitted in a wireless communication system based on OFDM.
2. Description of the Related Art
Research is ongoing on an Orthogonal Frequency Division Multiplexing (OFDM) scheme useful for high-speed data transmission in a radio channel of a mobile communication system.
An OFDM scheme for transmitting multi-carrier data is a type of multi-carrier modulation scheme. In an OFDM scheme a serial to parallel conversion process is performed for a symbol stream. The parallel signals are then modulated into multiple orthogonal sub-carriers such as multiple orthogonal sub-carrier channels. The orthogonal sub-carrier channels are then transmitted.
FIG. 1 is a block diagram illustrating a structure of a transmitter for a conventional OFDM system.
Referring to FIG. 1, the OFDM transmitter is provided with an encoder 101, a modulator 102, a Serial to Parallel Converter (SPC) 103, an Inverse Fast Fourier Transform (IFFT) processor 104, a Parallel to Serial Converter (PSC) 105, and a Cyclic Prefix (CP) inserter 106.
The encoder 101 is a channel-coding device. The encoder 101 receives an information bit stream and performs a channel coding process for the received information bit stream.
Conventionally, the encoder 101 uses either a convolutional encoder, a turbo encoder, a Low Density Parity Check (LDPC) encoder, or the like.
The modulator 102 performs a modulation process such as Quadrature Phase Shift Keying (QPSK), 8-Phase Shift Keying (8PSK), 16-Quadrature Amplitude Modulation (16QAM), or the like.
Although not illustrated in FIG. 1, a rate matcher could be inserted to perform repetition or puncturing between the encoder 101 and the modulator 102. The SPC 103 receives the output of the modulator 102 and generates parallel signals.
The IFFT processor 104 receives the output of the SPC 103 and performs an IFFT process. The PSC 105 serially converts the output of the IFFT processor 104. The CP inserter 106 inserts a CP into an output signal of the PSC 105.
In the IFFT processor 104, frequency domain data is input and time domain data is output. Because the conventional OFDM system processes input data in the frequency domain, there is a drawback in that a Peak to Average Power Ratio (PAPR) increases when the IFFT processor 104 transforms the frequency domain data into the time domain data.
The PAPR is an important factor considered in an uplink transmission. When the PAPR value increases, the cell coverage decreases. Efforts for reducing the PAPR value have focused on the uplink transmission so as to not increase the cost of terminals for the system. In the uplink transmission based on OFDM, a multiplexing scheme that is modified from the conventional OFDM scheme can be exploited. That is, a method can be exploited which can process data in the time domain without processing data in the frequency domain. For example, data could be processed in the time domain without a channel coding or modulation.
FIG. 2 is a block diagram illustrating a transmitter of the OFDM system based on the modified uplink transmission scheme.
Referring to FIG. 2, the OFDM transmitter is provided with an encoder 201, a modulator 202, an SPC 203, a Fast Fourier Transform (FFT) processor 204, a mapper 205, an IFFT processor 206, a PSC 207, and a CP inserter 208.
The encoder 201 receives an information bit stream and performs a channel coding process for the received information bit stream. The modulator 202 performs a modulation process such as QPSK, 8PSK, 16QAM, or the like. As described above, a rate matcher could be inserted between the encoder 201 and the modulator 202 of FIG. 2. The SPC 203 receives the output of the modulator 202 and generates parallel signals. The FFT processor 204 receives the output of the SPC 203 and performs an FFT process. The mapper 205 maps the output of the FFT processor 204 to the input of the IFFT processor 206. The IFFT processor 206 performs an IFFT process. The PSC 207 serially converts the output of the IFFT processor 206. The CP inserter 208 inserts a CP into an output signal of the PSC 207.
FIG. 3 is a block diagram illustrating an operation of the mapper of FIG. 2. The operation of the mapper will be described with reference to FIG. 3.
Data symbols 301 for which the channel coding or modulation has been performed are input to an FFT processor 204. The output of the FFT processor 204 is mapped by the mapper 205 (not shown) before being input to the IFFT processor 206. The output 305 of the IFFT processor 206 is input to the PSC 207.
The mapper 205 maps a signal 303 that was transformed from the time domain to the frequency domain by the FFT processor 204 to an input position of the IFFT processor 304, such that the signal 303 could be carried on a proper sub-carrier.
When the output of the FFT processor 204 is successively mapped to an input part of the IFFT processor 206 in the mapping process, successive sub-carriers are used on the frequency domain. This is referred to as Localized Frequency Division Multiple Access (LFDMA).
Furthermore, when the output of the FFT processor 204 is mapped to the input part of the IFFT processor 206 while maintained in an arbitrary equal interval, equal-interval sub-carriers are used on the frequency domain. This is referred to as Interleaved Frequency Division Multiple Access (IFDMA) or Distributed Frequency Division Multiple Access (DFDMA). Hereinafter, both the IFDMA and DFDMA are referred to as the DFDMA.
FIG. 4 illustrates a comparison between positions of sub-carriers of the DFDMA and LFDMA in the frequency domain.
As indicated by reference numeral 401 of FIG. 4, sub-carriers for terminals using the DFDMA are positioned in an equal interval in the entire frequency domain. As indicated by reference numeral 402 of FIG. 4, sub-carriers for terminals using the LFDMA are successively positioned in a portion of the frequency domain.
The LFDMA and DFDMA schemes have the following unique characteristics.
By exploiting a partial frequency bandwidth with successive sub-carriers for the entire system frequency bandwidth, the LFDMA scheme can select the partial frequency bandwidth with a high channel gain in a frequency selective channel whose variation is significant in the frequency bandwidth and then transmit data through the selected bandwidth. Thereby frequency scheduling gain is achieved.
On the other hand, the DFDMA scheme can obtain various channel gains by exploiting multiple sub-carriers distributed over a wide bandwidth, thereby obtaining frequency diversity gain.
Thus, if channel gains on a frequency bandwidth-by-frequency bandwidth basis are known by a base station, frequency scheduling can be first be considered for an uplink transmission of a slow terminal. Thereby, better performance can be achieved when LFDMA is used. Even when channel gains on the frequency bandwidth-by-frequency bandwidth basis are not known by the base station, better performance can be achieved by exploiting DFDMA. Whereby, DFDMA is capable of increasing the frequency diversity gain in an uplink transmission of a fast terminal for which channel gain information is incorrect.
In the uplink transmission, the system allocates some resources to a terminal of the uplink transmission through uplink resource scheduling and allows the terminal to perform the uplink transmission. According to a resource scheduling and allocation method, the performance at an uplink transmission time is affected.
To increase the uplink performance as described above, uplink resources are allocated to a terminal capable of increasing the frequency diversity gain such that the DFDMA scheme is used, and are allocated to a terminal capable of increasing the frequency scheduling gain such that the LFDMA scheme is used. Accordingly, the base station must make due consideration in scheduling for the selection of DFDMA or LFDMA according to each terminal.
Accordingly, there is a need for a method and apparatus that can efficiently allocate resources at an uplink transmission time by mixing a Distributed Frequency Division Multiple Access (DFDMA) scheme and a Localized Frequency Division Multiple Access (LFDMA) scheme.